Use pkg/webhook along with build webhook implementation (knative#379)

* Add validation & defaut for btmpl

* add validation for cluster build template

* Add validation and defaults for build

* Use pkg webhook

* compiles

* Add webhook package

* Move tests

* Add tests

* webhook test

* cleanup unused code

* Revert previous change

* Add e2e tests for build with template and cluster template

* Fix typo

* Fix test typo

* Increase coverage

* Address comments

* Add tests

* Rename file to generic name